The service function of maintaining and making available the specialized rat strains developed by the late Drs. T.F. and L.M. Zucker is to be continued. In addition to the well-publicized 13M, with its "fatty" mutant (congenital obesity), 4STL (large body size), and 4STS (small body size), are also available. These stocks are random-bred within closed colonies, thereby retaining their original qualities without loss of vigor from inbreeding. Studies of the mode of action of the fatty gene continue, interest centering on the fatty's amino acid metabolism, particularly that of lysine and glycine. The effects of lysine and glycine supplemenation and deficiency, upon growith and obesity, are being investigated; also the quantitative and qualitative status of the fatty's connective tissue.